The endogenous viruses of the guinea pig include the B-type GPV, the intracisternal A particle found predominantly in two transplantable leukemias, and the guinea pig herpes virus. We are currently characterizing the structure of GPV, purifying large amounts of virus, and developing probes for the study of levels of GPV RNA and protein synthesis in normal and malignant guinea pig cells. In attempting to immortalize the tumor cells of transplantable guinea pig leukemias, we could show that both herpes saimiri virus and Epstein-Barr Virus (EBV) were infectious for both normal guinea pig lymphocytes and leukemic tumor cells. We also were able to observe and partially characterize two new oncornaviruses by electron microscopy. BIBLIOGRAPHIC REFERENCES: Michalides, R., Schlom, J., Pearson, J., Perk, K. and Dahlberg, J.: Characterization of the oncornavirus particles in the plasma of guinea pigs with the L2C leukemia. J. Virol. 18: 1120-1130, 1976. Papas, T., Dahlberg, J. and Sonstegard, R.: Presence of C-type particles in pike lymphoma. Nature 261: 506-509, 1976.